Many methods and apparatus for manufacturing various articles from hydraulic cement and similar materials have been proposed in the past and has been used in many fields. According to these prior art methods a mixture of the castable substance is cast into an open mould under atmospheric pressure and after the cast substance has been hardened it is taken out of the mould. To increase the density of the cast substance, oscillation, compressive force or centrifugal force is applied to the cast substance or the surface of the cast article is subjected to a reduced pressure. Even when any one or combinations of these treatments are used, it is not always possible to obtain products having desired density and mechanical strength. More particularly, in order to prepare a desired mixture it is necessary to thoroughly admix the raw materials and in order to satisfactory cast the mixture thus prepared it is necessary to add an excess quantity of water to the raw materials. Even when minute care is taken during the mixing operation, it is unavoidable to prevent air bubbles from becoming entrained in the mixture. Such excessive water or entrained air results in air voids in the body of the cast article or causes it to shrink. Where curing heat is applied, the excess water is vapourized, forming large voids in the cast article. Also the entrained air increases its volume when heated thus also forming large voids which often interconnect with each other to form cracks. Although many efforts have been made to remove such excess water or entrained air, it has been difficult to completely remove them so that it has been difficult to obtain dense, crack and void free products. For this reason, for finishing the surface of the cast products as described above it is necessary to let them stand for one or two hours for the purpose of stabilizing the composition of the cast mixture. During this interval, the excess water oozes from the cast product and collects on the surface thereof. Unless such oozed water is completely removed, surface finishing or post treatment should not be made. For this reason, the casting process of the mixture requires a considerable amount of labor and time to obtain finished products of desired characteristics which are not suitable for present day mass production which requires high speed production at low cost. The post treatment including vibration or centrifugal force produces noise, thus causing public hazards. Also the application of a pressure or a reduced pressure can only improve the quality of the surface portion of the cast product and can not improve the quality of the deep portion of the core of the article. Of course these post treatments require special equipment, skill and additional labor and cost.